1. Field of the Invention
This invention relates to a process for controlling load distribution to make a distribution ratio of rolling loads between different stands coincide with a preset value in a continuous rolling mill.
2. Description of the Prior Art
In a continuous rolling mill, it is strongly required that rolling loads to different stands stay between upper and lower limits provided for according to ratings of machines or appliances, and additionally, that the rolling load distribution ratio among the stands be coincident with a separately determined appropriate preset value.
FIG. 2 illustrates a conventional load distribution control of this type which is proposed, for example, by a Japanese Patent Application No. 58-192738. The arrangement includes a plurality of rolling stands #4, #5 and #6 which in turn include roll pairs 2a-3a, 2b-3b and 2c-3c for rolling a work 1 to be rolled, rolling reduction position controlling devices 4a, 4b and 4c and rolling load detecting devices 5a, 5b and 5c. Thus, rolling loads at the individual stands are detected by the load detecting devices 5a, 5b and 5c are compared with a set value of load distribution ratio in a load distribution controlling device 7 in order to change material thickness rolling reduction values for the individual stands in accordance with the results of such comparison, and at the same time, rolling reduction position correcting amounts .DELTA.S4, .DELTA.S5 and .DELTA.S6 for the individual stands are calculated on condition that, as for a material thickness on the exit side of the last stand, a value before load distribution correcting control is maintained. Then, in accordance with the results of such calculations, instructions for changing are outputted to the rolling reduction position controlling devices 4a, 4b and 4c of the stands at such timings that a rolling reduction position changing point on the plate material by an upstream side stand reaches a downstream side stand.
Operations will be described now. It is assumed that stand rolling loads from the load detecting devices 5a, 5b and 5c are P4, P5 and P6. If set ratio values for them are C4, C5 and C6, then EQU P4:P5:P6=C4:C5:C6 (1)
For this equation, an error between the two downstream side stands #5 and #6 is estimated EQU .DELTA.err5=C5.multidot.P6-C6.multidot.P (2)
In order to make a load distribution ratio between the two stands coincide with C5:C6, the material thickness at the #5 stand is changed by .DELTA.h5.sup.L to vary P5 and P6 as follows: EQU .DELTA.P5=-Q5.multidot..DELTA.h5.sup.L ( 3) EQU .DELTA.P6=Q6.multidot..DELTA.h5.sup.L 4)
Here, ##EQU1## EQU .DELTA.5=C5.multidot.Q6+C6.multidot.Q (6)
Here, Q5 and Q6 are influence coefficients which is had on the rolling loads P5 and P6 by a change .DELTA.h5 of the material thickness, and substantially accurate values can be obtained therefor by a theoretical expression or by an experimental expression.
Meanwhile, rolling reduction correcting amounts .DELTA.S5 and .DELTA.S6 for the #5 and #6 stands to realize a material thickness variation .DELTA.h5 on condition that the material thickness h4 on the exit side of the #4 stand and the material thickness 16 on the exit side of the #6 stand remain unchanged are calculated by ##EQU2## where mi is a mill spring constant at each stand and qi is a plasticity coefficient of the material of the work.
Such rolling reduction position correcting operations are carried out after necessary timing adjustments which involve consideration of travelling of the work.
In the load distribution control of the prior art, calculations of material thickness changing amounts and rolling reduction position correcting amounts use ideal coefficient values obtainable from rolling phenomenon model expressions which can be found out in advance as described hereinabove. In fact, however, due to inaccuracy of such coefficient values even if rolling reduction position correction depending upon a single load distribution correction calculation is performed for a given stand, fluctuation will be caused in a material thickness value on the exit side thereof and hence a rolling load change by a thickness control may be produced or a rolling load amount after changing at each stand may not coincide with a set value therefor, resulting in the necessity of performing another load distribution control. As a result of requirement of a time, longer than necessary, for convergence of the rolling load amounts to an appropriate ratio, portions having no accurate material thickness and having unacceptable strip shape will remain in a product, resulting in a great economical loss.
This originates from the fact that there exist some errors in various constants used for calculations in a load distribution controlling device and also in characteristics of an actual material being rolled, and thus, there is a room for improvement of such a load distribution controlling device.